1. Field of the Invention
The present invention relates to a motor driving system, for driving a motor, using a power semiconductor module formed, by wiring, on a power semiconductor chip.
2. Description of the Related Art
A drive circuit, to drive a power device such as a motor, employs a power semiconductor module having a power semiconductor chip to control the drive current. The power semiconductor module having the power semiconductor chip controls the drive current by being energized intermittently. After the long intermittent energizing operation, the difference in the coefficient of linear expansion between the electrode and the wire, etc. and the repeated sharp temperature change during the drive operation combine to generate a thermal stress in the wire connection boundary. Especially, the larger wire diameter, the larger wire rigidity so that a conspicuous thermal stress in the wire connection boundary is generated. Thus, the wire is liable to be separated or otherwise damaged at the connection boundary, thereby posing the problem of a reduced endurance to power cycles and heat cycles (See Japanese Unexamined Patent Publication No. 2003-303845, for example).
In the case where the connecting surface (electrode surface), between the semiconductor chip and the main circuit wire, connected in the power semiconductor module is degenerated to separate the main circuit wire from the electrode of the semiconductor chip as described above, the power supply to the power devices would fail.
The separation of the wire from the electrode depends on the amount and frequency of temperature change of the semiconductor chip on the one hand and the intermittently energizing condition and the operating environment on the other hand. Therefore, the separation of the wire is affected considerably by the actual operation of the user and the time of separation is difficult to predict.
In view of this, according to the prior art, the service life is calculated on the assumption that the power semiconductor chip is used under a maximum load in a predetermined cycle of operation.
In the case where the service life of the power semiconductor module is calculated based on a maximum load imposed on the power semiconductor chip as described above, it may happen that the power semiconductor chip is replaced in spite of the fact that the actual load is lighter than the assumed load or power cycle and the power semiconductor chip is still usable. In the case where the load actually imposed is heavier than the assumed load or power cycle, on the contrary, the power semiconductor chip may be damaged before being replaced at the time of expiry of the calculated service life.
Therefore, it is desirable that the service life in terms of power cycles of the power semiconductor chip is accurately determined and the power semiconductor module is effectively used to the end of the semiconductor life while being replaced before any fault which may occur, thereby assuring satisfactory preventive maintenance.
Accordingly, it is an object of this invention to predict the separation of the main circuit wire and prevent the operation failure of the power semiconductor module, and the motor driving system using the power semiconductor module, which otherwise might be caused by the separation of the main circuit wire.